The present invention relates to servo-actuators for providing mechanical output such as rotation of a shaft in response to energization of an electric drive motor. The invention relates particularly to rotary actuators driven by a low voltage motor such as a small relatively high RPM motor of sub-fractional horsepower operating from a vehicle on-board power supply, typically in the range 12-24 volts. Electrically operated servo-actuators have been found desired for various on-board vehicle applications.
In one application it has become desired to operate a vehicle engine air throttle valve, typically of the butterfly type, by an electrically operated servo-actuator in place of a mechanical linkage connected to a throttle pedal moved directly by the vehicle operator. This type of arrangement is known as a "drive-by-wire" system inasmuch as the vehicle throttle pedal employs an electrical transducer which provides an electrical signal to an electronic controller which provides an output driver signal to a motorized actuator for moving the throttle butterfly. Such arrangements have the advantage that the electronic controller can override the throttle pedal signal from the vehicle operator in the event that vehicle operation is jeopardized or the operator called-for engine operation would result in a prohibitive amount of exhaust emissions or loss of wheel traction. In addition, electrically operating the throttle directly with a motorized actuator eliminates the need for a separate throttle actuator for cruise control mode of operation of the vehicle.
In providing an electrically operated throttle servo-actuator for motor vehicle applications, it is necessary to provide for automatic throttle return or closing. In particular, it is desired to provide for automatic or self return of the servo-actuator to an initial reduced opening or closed throttle position in the event of failure of the servo-actuator motor.
Typically, the throttle servo-actuator may be spring biased to a return or closed throttle position in the event of electric motor failure. However, in order to provide sufficient torque to reverse drive the entire actuator mechanism, for example, a gear reduction train, a significant amount of force is required by the return spring to overcome the friction of the actuator drive mechanism. This results in additional output power requirements for the servo-actuator in order to deflect or wind up the return spring in addition to moving the throttle.
In another application, it has been desired to electrically operate a valve for controlling flow of heated engine coolant to the heater core for heating the air in a vehicle passenger compartment. In such an application, it is desired that a numerically high ratio of speed reduction be employed for precise or fine control of the valve butterfly position. In order to accomplish such control, a substantial torque output is required from a relatively small motor drawing low current i.e., about 10 amperes or less when operating from a supply of 12 to 24 volts. This torque requirement makes a worm drive the preferred arrangement for providing high reduction at a minimum number of gears and minimum cost. However, in the event of motor failure during operation, it is required that the valve be biased to the full open position to provide maximum heating of the passenger compartment in order to protect the vehicle occupants from exposure to extreme cold.
Where a worm driven gear train is provided for an application for rotary actuation, it is virtually impossible to reverse drive the gear train with a spring because torque applied to the driven gear meshing with the worm will not reverse drive the worm for any practical helical angle of the worm suitable for providing the desired gear reduction.
Accordingly, it has been desired to provide a way or means of utilizing a motorized worm for driving a gear train in a servo-actuator to provide a rotary output such as for controlling the opening and closing of the vehicle throttle or heater core valve in response to electrical control signals and to provide for a release and self return to a desired position in the event of failure of the drive motor.